The present invention is in the field of thermoplastic coatings, more specifically polyethylene coatings for ferrous metals.
Plastics in the form of powder have been used for coating metal substrates to protect them from corrosion. Various methods of application and curing are known such as preheating an object to a temperature ranging from 350.degree. to 600.degree. F and dipping the object in a fluidized bed, spraying it by flock or electrostatic gun. No one type of polyethylene by itself has proven to be a satisfactory coating material due to poor adhesion to the metal substrate of low density types or the high cost and poor physical characteristics of the high density types. To obtain an acceptable adhesion level, primers were developed consisting of either special high density polyethylene or ethylene copolymers. These primers may be applied to the metal substrate in relatively thin films (2-4 mils), fused and subsequently covered with a finish coat of low density polyethylene to develop the total thickness required. This two-step application procedure requires close temperature control of both application steps with concomitant high capital cost for production equipment.
The use of primers as a full thickness coating has not proven satisfactory for several reasons. Pigmentation to provide resistance to ultra violet rays may adversely change adhesive characteristics. Cured physical properties are unsatisfactory in that the surface may be quite soft and rubbery or hard and brittle. Also, the high density primer is much more expensive than low density polyethylene.